The present invention pertains generally to active noise canceling microphones and related devices. More particularly, the present invention relates to a method for designing an acoustically motivated housing and architecture for an active noise canceling microphone comprising two microphone elements, an analog or digital or hybrid (analog and digital) control circuitry and associated control codes or software. The new acoustic housing design method provides improved background noise canceling and enhanced speech intelligibility for such an active noise canceling microphone system as described herein. The performance improvement is realized due to the optimal acoustic design of the shape and dimensions of the microphone housing and the unique assembly method of the microphone elements inside the housing.
Noise canceling microphones are widely used in commercial, industry, and military applications where clear communication in noisy ambient environments is required. There are basically two types of noise canceling microphone designs. A passive noise canceling microphone typically incorporates a single membrane to sense ambient sound, where the housing of that membrane is open to the environment on both sides. Far-field sounds impact the membrane essentially equally on both sides, generating little net movement (particularly at low frequencies), and thus a low sensitivity. Near-field sounds (such as speech when the microphone is placed close to a speaker's mouth) cause the membrane to move significantly in one direction over another, thus causing a higher near-field sensitivity. This higher sensitivity to close-range voice versus lower sensitivity to far-field ambient noise provides a low frequency improvement in the signal-to-noise ratio because of the associated far-field noise rejection, thus improving low frequency speech intelligibility.
The case, or housing design, for passive noise canceling microphones usually concerns housing a single microphone element and providing for the ventilation of both sides of the membranes is discussed in U.S. Pat. Nos. 5,442,713, 5,854,848 and 6,009,184. The invention described herein is different from this prior art since it is related to a unique housing design for active noise canceling microphones using two omni-directional microphone elements.
U.S. Pat. No. 5,854,848 and U.S. Pat. No. 6,009,184, issued to Tate et. al. describe a noise control device for a boom mounted passive noise canceling microphone. This device utilizes a curved reflector attaching at the back surface of the microphone housing facing away from the desired signal source, or speaker's mouth. This prior art is shown to be effective for passive noise canceling microphones that reduce low frequency noise much more effectively than high frequency noise. It does not necessarily work for active noise canceling microphones since the effectiveness of the active element will be highly dependent on the broadband coherence between the two individual microphone elements. The addition of such a reflector on one side of the microphone housing as described by Tate will inevitably degrade the coherence between the two microphone elements especially at high frequencies. This may instead result in a degradation of the performance of the active noise canceling microphone.
Active noise canceling microphones typically utilize two individual microphone elements (preferably omni-directional electret microphones) and an active element such as a subtraction circuit is employed in order to electronically difference the two microphone signals. The two microphone elements are disposed such that a first microphone element receives the desired speech input and the background noise present in the vicinity of the speech, and a second microphone element senses substantially only the background noise. Therefore, a noise reduced speech signal can be generated when subtracting the second microphone signal from the first microphone signal by the active element of the active noise canceling microphone. The noise canceling performance of such an active noise canceling microphone is highly dependant on the broad band coherence between the two microphone elements. In addition, the level of the speech signal in the final output of such an active noise canceling microphone is directly related to the amplitude difference of the speech signals sensed by the two microphone elements.
U.S. Pat. No. 5,917,921 issued to Sasaki et. al., discusses the use of two microphone elements to form an active noise reducing microphone apparatus having an adaptive noise canceller. The Sasaki patent teaches that the two microphone units should be disposed in proximate locations, being oriented in the same direction or alternatively in opposite directions under certain circumstances. However, the Sasaki patent does not disclose or teach the effects of the microphone shape and dimensions on the coherence function and the amplitude difference in the desired signal sensed by the two microphone elements. These effects are very important in terms of the noise canceling performance and speech intelligibility achievable by the active noise canceling microphone apparatus. Secondly, the active noise canceling apparatus with two microphone elements facing the same or opposite directions taught in the Sasaki patent reduces primarily the low frequency wind noise. In an attempt to reduce the broadband background noise, much more strict constraints are required on the distance between the two microphone elements and the design of the acoustic baffle separating the two elements. And furthermore, the configuration of orienting the two microphone elements in the same direction is not a practical choice since such a configuration may result in a more effective speech canceller than a noise canceller.
U.S. Pat. No. 5,673,325 issued to Andrea describes an active noise canceling microphone for use with a telephone handset or a boom microphone device. This active noise canceling microphone again consists of two individual microphone elements arranged such that one microphone receives both the desired speech input and the background noise while the other microphone receives substantially only the background noise. The Andrea patent teaches that a small distance (preferably 0) between the two microphone elements is required to obtain good noise canceling performance. On the other hand, in order to prevent the active circuit from canceling the desired speech signal, an acoustic baffle is needed between the two microphone elements. However, the Andrea patent does not teach a specific size or shape of the acoustic baffle design so that both good noise canceling performance for background noise and a significant differentiation in near-field speech (desired signal) amplitudes between the two microphone elements can be achieved.
In summary, this review of the prior art in housing design and microphone architecture for active noise canceling microphones does not teach the importance of the housing shape and dimensions as these attributes relate to the performance of the active noise canceling microphone. What would be useful is a method of designing an acoustic baffle that improves the noise canceling performance of an active noise canceling microphone.